Hydraulic radial piston motors having an outer cam ring and outwardly working pistons, which are in contact with the cam surface of the cam ring via rotating cam rolls, are available in two main types, on the one hand a type in which the cam roll is mounted in roller bearings on the end of a shaft connected to the piston and, on the other hand, a type in which the cam roll is slidably arranged directly against the piston in a recess thereof.
The last-mentioned type, which corresponds to the one accounted for in the introductory part, has a number of advantages both in respect of size and cost by the height of the assembled piston and cam roll being considerably smaller than that of the first-mentioned type, which means that the motor can be made smaller and more compact with maintained output.
However there are also problems and drawbacks in connection with radial piston motors with slidably mounted cam rolls. For instance, the frictional losses between piston and cam roll are relatively great. These losses are about four times greater than the corresponding frictional losses between piston and cylinder wall. These great frictional forces imply considerable wear on the slide bearing surfaces and, consequently, a risk of seizing therebetween.
The seizing risk is dependent not only on the great forces which are transmitted between cam ring and cylinder block, but also on the varying reaction force exerted by the cam ring. The undulated cam surface thereof in fact varies by an angle between 0.degree. and a maximum of about 45.degree. in relation to the tangential direction of the can ring and the cylinder block. Correspondingly, the reaction force exerted by the cam ring on the piston and the cylinder block will vary between 0.degree. (i.e. radially inwards to the center of rotation) and about 45.degree.. This variation of forces has up to now made it impossible to form a constant oil film thickness between the bearing surfaces between cam roll and piston, and piston and cylinder wall, respectively. The variation in forces will also during a certain part of the working stroke of the piston coincide with the direction of rotation of the cam roll, implying that the cam roll, mechanically seen, acts as having a very low speed or even "standing still" in relation to the bearing surface of the piston. This further reduces the possibilities of forming a constant oil film thickness. A varying thickness of the oil film in turn results in an increased metallic contact between the bearing surfaces, which causes increased wear, a greater risk of seizing, greater frictional forces and reduced efficiency.
Pistons with slidably mounted cam rolls are also expensive to manufacture owing to the complicated shape, which requires precision machining in several steps.
Owing to the small material thickness in the lateral portions of the piston and the great forces acting on these, it is also necessary that the entire piston be made of a high-quality material.